Mass memory components such as floppy discs, CD ROMs, and other similar memory components are often found in battery powered portable devices. Rotation of these mass memory components are performed using electric motors which are electronically switched by integrated drive systems.
A prior art diagram for a typical current mode PWM switching drive system is shown in FIG. 1. A generic actuator or external load in the illustrated example is a Voice Coil Motor (VCM). A current control loop is formed at sum node A where a balance occurs between two separate currents applied to this node A. One current is provided by the control voltage Vdac through connecting resistor R1. The other current is provided by the voltage Vsense through connecting resistor R2. The voltage Vsense is provided as an output of a sense amplifier. This sense amplifier portion of the drive system is referred to as a current sense circuit.
The loop further comprises a differential error amplifier coupled between the reference voltage Vref and the voltage present at node A. The feedback voltage closing the control loop is derived from the voltage drop on a sense resistor R3 connected in series with the load VCM.
It is well known in the art that if the differential voltage across the R3 resistor (i.e., Vin1-Vin2) is equal to the level of the reference voltage Vref in the resistive series Ra, Rb, then a current of a relatively small value is provided. Therefore, an eventual mismatching between the resistive ratios Ra/Ra' and Rb/Rb' will not cause sensing inaccuracies in the control system. In contrast, if the differential voltage across the sensing resistor R3 is significantly different from the reference voltage, a mismatching of the actual values of the above mentioned resistors will introduce sensing inaccuracies.
In a typical current mode PWM switching drive system like the one depicted in FIG. 1, the motor's winding is driven through a single bridge with a supply voltage that is typically 12V. Also, the circuitry that forms the control loop is supplied at 12V. The current sense amplifier thus receives input signals ranging between 0 and 12V, and its output is typically half of the supply voltage.
The current sense amplifier is formed using a feedback operational amplifier OP for carrying out a conversion of the differential signal into a "single-ended" output signal Vsense. Consequently, a problem exists when the output voltage Vsense is significantly different with respect to the reference voltage Vref. Such an offset in voltage is caused by mismatching of resistances.
Labeling V.sup.+ as the voltage on the positive terminal, the current through Rb is provided by EQU I=(V.sup.+ -Vref)/Rb.
When there is an offset-free operational amplifier, the voltage on the negative terminal is the same. In absence of an input differential signal, the current that flows in the two branches is the same, hence the output voltage is given by EQU Vo=V.sup.- -I*Rb'=V.sup.+ -(V.sup.+ -Vref)*(Rb'/Rb).
If the matching of resistances are equal (Rb=Rb'), then the output voltage is equal to the reference voltage. Assuming a resistive mismatching (Rb=R, Rb'=R+.DELTA.R), the output voltage is equal to EQU Vo=V.sup.- -I*Rb'=V.sup.+ -(V.sup.+ -Vrif)*(1+.DELTA.R/R).
For a best case situation, if there is a matching within 1% of the resistances and a difference of 6V between the common mode signal at the bridge terminals and the reference voltage, then the voltage offset for Vsense is about 60 mV.
Therefore, there is a need to reduce voltage consumption and the size of integrated current mode drive systems. This need is driven by the increased supply voltages applied to these devices, which in turn imposes a scaling down of the supply voltages for the desired loads, as well as the size of the integrated devices themselves.
However, the scaling process provides a voltage difference between the reference voltage Vref and the differential voltage present at the inputs of the current sense amplifier. This difference leads to sensing inaccuracies because of the increase of the difference between the level of the common mode signal on the bridge terminals and the reference voltage level.